We have shown in previous work that the late-time (nebular) spectra of SNe Ia,
which probe the inner ejecta, may more easily yield information about the
physics of the explosion and thus, more or less directly, those of the
progenitors [1]}. One striking feature distinguishing
low-luminosity SNe Ia from normal ones at late times is the dominance of
[Fe iii] emission at the lowest velocities. These lines are also present in
“normal” SNe Ia, but they typically coexist with [Fe ii] lines. This
unexpected property was observed in SN 1991bg, where [Fe iii] lines dominate at
epochs beyond 200 days and velocities below 3000 km s\(^{-1}\)[2]}, [1]}, and similarly in the 91bg-like SN 1999by
[4]}. They are also strong in SN 2003hv
[5]}, [6]} and in SN 2007on, where a complex spectrum
requires two components with different - opposite - line-of-sight velocities
[7]}. At the low densities of the late-time SN ejecta (at epochs
exceeding 300 days for normal SNe, but as early as 200 days for sub-luminous
ones), a high ionization is less the result of a high temperature than it is of
low density, which suppresses recombination more efficiently than high
temperature favours it. Therefore, if low-luminosity SNe Ia show a higher
degree of ionization at late times, this is likely to indicate a low central
density. These SNe are also typically characterised by lower velocity ejecta at
early times [8]}. The combination of these factors, as well as the
early transition to the nebular regime, suggests that low-luminosity SNe Ia are
the explosion of progenitors of lower mass than normal SNe Ia. Their
progenitors are likely to be white dwarfs that did not reach M\(_{\textrm {Ch}}\)  when they
exploded. Such white dwarfs can only be exploded by compression, in violent
events such as mergers or collisions of two white dwarfs.
